The efficiency of solar-pumped lasers (SPLs) is limited when the length of the laser medium is unsuitable. This is because superfluous regions in the laser medium introduce losses and contribute slightly to the stimulation of radiation in the laser resonator. Before designing an SPL, an appropriate length of laser medium is critical. We present a method to calculate the optimal length of the gain medium in SPLs by exploring the relationship between the absorbed solar power and material loss for different laser medium lengths. Thus, the lengths of Nd:YAG crystals with diameters of 3 to 6 mm were optimized, and the output characteristics were calculated numerically. The maximum collection efficiency (CE) (40.1  W  /  m²) was obtained for the 5.5-mm diameter Nd:YAG crystal rod of length 21.1 mm, which was 1.7  W  /  m² higher than the previous numerical record. The optimal length of the 6-mm diameter Nd:YAG crystal rod was found to be 21.9 mm. For a laser rod of this length, a CE of 36.3  W  /  m² is expected. This value is 1.13 times greater than the existing experimental record for the Nd:YAG crystal of the same diameter, which highlights the importance of optimizing the length of the laser rod.